earth science

Geological compass

Geological compass is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Geological compass rather than just read about it. In short: There are a number of different specialized magnetic compasses used by geologists to measure orientation of geological structures, as they map in the field, to analyze and document the geometry of bedding planes, joints, and/or metamorphic foliations and lineations. In this aspect the most common device used to date is the analogue compass.

Key takeaways

Geological compass belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Geological compass to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Geological compass from memory before moving on to harder problems.

Reference excerpt

There are a number of different specialized magnetic compasses used by geologists to measure orientation of geological structures, as they map in the field, to analyze and document the geometry of bedding planes, joints, and/or metamorphic foliations and lineations. In this aspect the most common device used to date is the analogue compass.

Classic geological compasses Classic geological compasses that are of practical use combine two functions, direction finding and navigation (especially in remote areas), and the ability to measure strike and dip of bedding surfaces and/or metamorphic foliation planes. Structural geologists (i.e. those concerned with geometry and the pattern of relative movement) also have a need to measure the plunge and plunge direction of lineations. Compasses in common use include the Brunton compass and the Silva compass.

Modern geological compasses

The concept of modern geological compass was developed by Eberhard Clar of the University of Vienna during his work as structural geologist, which he published it in 1954. An advantage of his concept is that strike and dip is measured in one step, using the vertical circle for dip angle and the compass for the strike direction. The first implementation was by the VEB Freiberger Präzisionsmechanik in Freiberg, Germany. The details of the design were made in a close cooperation with the Freiberg University of Mining and Technology. In 2016 Brunton Inc. introduced the Axis Pocket Transit which, for the first time, offered simultaneous measurements of both strike and dip and trend and plunge in a variety of configurations. It featured an unconventional lid design that swung a full 360 degrees in both directions and two axes that allow precise measurement of vertical and horizontal angles on all configurations of bedding surfaces.

Usage

Geological compasses are distinctive because of the anti-clockwise direction of the numbers on the compass dial. This is because the compass is used to determine dip and dip-direction of surfaces (foliations), and plunge and plunge-direction of lines (lineations). To use the compass one aligns the lid of the compass with the orientation of the surface to be measured (to obtain dip and dip direction), or the edge of the lid of the compass with the orientation of the line (to obtain plunge and plunge direction). The compass must be twisted so that the base of the compass becomes horizontal, as accomplished using the spirit level incorporated in it. The needle of the compass is then freed by using the side button, and allowed to spin until the damping action slows its movement, and then stabilises. The side button is released and the needle is then firmly held in place, allowing the user thereafter to conveniently read the orientation measured. One first reads the scale that shows the angle subtended by the lid of the compass, and then depending on the colour shown (red or black) the end of the compass needle with the corresponding colour. Data are then recorded as (for example) 25°->333° (dip and dip-direction) or (plunge and plunge-direction). This compass has the most use by structural geologists, measuring foliation and lineation in metamorphic rocks, or faults and joints in mining areas.

Digital compasses With the advent of the smartphone, geological compass programs based on the 3-axis teslameter and the 3-axis accelerometer have also begun to appear. These compass programs use vector algebra to compute plane and lineation orientations from the accelerometer and magnetometer data, and permit rapid collection of many measurements. However, some problems are potentially present. Measurements made by smartphone geological compasses can potentially be susceptible to noise, mainly due to vibration or rapid hand movement. Users of a smartphone compass should carefully calibrate their devices and run several tests against traditional magnetic compasses in order to understand the limitations of their chosen program. With traditional compasses there is no record of error caused by poor damping or operator movement. This limitation is removed by use of a digital compass, though these may be more error prone because of the sensitivity of the accelerometer, which programs use to determine vertical and horizontal. Therefore, professional use of a digital geological compass requires the recoding of variance in individual measurements. There is no data that suggests digital compasses are subject to any measurable form of magnetic disturbance. Modern remote sensing techniques as LiDAR and photogrammetry allow to obtain accurate and dense 3D point clouds. These point clouds allow the measurement of orientations of planar surfaces. Jordá et al. performed a comparison of the orientations of discontinuities measured by means of classical geological compass and a photogrammetric 3D point cloud demonstrating that remote sensing field discontinuity collection provides a reliable alternative to the use of geological compass.

References

External links Breithaupt Precision Instruments Geological Compass Freiberg Precision Instruments (FPM) Geological Compass Archived 2015-02-14 at the Wayback Machine Brunton Geological Compass

Illustrations

Worked examples

Example 1 — a first encounter with Geological compass

Start with the simplest possible case. Write down what Geological compass claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Geological compass before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Geological compass ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Geological compass

In research: Geological compass appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Geological compass in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Geological compass is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geological tools, so understanding it makes those chapters shorter.
In everyday life: Look for Geological compass outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Geological compass in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Geological compass means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Geological compass out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Geological compass in simple terms?

Why does Geological compass matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Geological compass?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Geological compass.